Plain English Meaning
A short-notice weather warning sent out when something dangerous is happening in the sky right now — bad enough that every pilot, in every kind of aircraft, needs to know about it immediately.
Full Definition
An unscheduled in-flight weather advisory issued by the Aviation Weather Center to alert pilots to hazardous conditions that pose an immediate threat to all aircraft. A UWS is issued when severe or extreme turbulence not associated with thunderstorms, severe icing not associated with thunderstorms, or widespread duststorms, sandstorms, or volcanic ash lowering to less than three miles is occurring or expected to occur. It carries the same as a but is issued on a more immediate, unscheduled basis when conditions warrant.